I inherited some pieces of turquoise jewelry from my grandmother. …they all have the letters FD etched into the silver on the back
It may possibly be David Frank. Having said that I can find no information about him anywhere. It may also be a previous owner.
Sorry, not much help.
I would agree that these marks could be from the previous owner. However, you inherited them from your grandmother who I am assuming does not have the initials FD. Did she tell you anything about them? Thanks for sharing.